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Community and Q&A

House fully insulated with 2″ rigid foam

beining35 | Posted in Green Building Techniques on

I have been an avid viewer of this site for a couple of years now, ever since I knew I was going to be building my new home. This is, however, my first post to the site.

I know that I am going to be asking questions that already have answers out there in other blogs on this site, but you all know that not every situation is the same so I would really like direct feedback on my exact situation so that I don’t miss anything. I am an extreme DIY’er that insists on doing something right the first time so that it lasts for decades, and I am always looking to do something new, especially if it can help my house be more efficient or ‘green’. I will have a general contractor for my house but I do plan to do a lot of work myself. My GC is on board for trying something new!

My questions today are all about exterior insulation, specifically about rigid foam.
I numbered each line so that maybe it would help you comment on specific areas of my plan.
First, a little about my situation:
1. My house will be built on about 6 acres in Northwest Ohio, so I have to worry about the cold and the hot (humidity). Climate zone 5 I believe.
2. My house will be about 2400 sqft. It will be a 2 story house with a full unfinished, walkout basement.

Here is my plan of attack. Please comment.

3. Poured concrete footers will be either sprayed with flexible moisture barrier or covered with plastic. This will stop moisture from seeping up through the footer into the walls.
4. 8″ concrete walls will be sprayed with flexible moisture barrier.
5. Walls will be 2×4 construction, 16” OC
6. I plan to cover the exterior with ½” OSB sheathing and plan it so that it ends flush with the outside of the foundation walls.
7. Wrap this sheathing with house wrap vapor barrier.
8. Use 2” of polyiso to cover all of the walls. It will be a continuous run of rigid foam from 2×4 walls down to the footer. Sealing all joints.
9. Install 1×4 latts 16” OC over top of the foam to act as nailers for the siding. Bottom part of the walls and foundation walls will be covered with stone veneer using a metal lathe and scratch coat. Between the siding and the stone there will be a drip edge and bug screen.
10. For the interior, I plan to install unfaced batt insulation in the 2×4 wall cavities.
1. How do I manage the transition from the wall to the roof/ceiling?
2. Best way to insulate attic/cathedral ceilings?
3. Should I worry about the siding warping or not laying straight when just being nailed to latts, vs sheathing?

Any advice will be much appreciated. Thanks for everyone’s help in advance!

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  1. iLikeDirt | | #1

    Others will expound on way more, but I'll just say that termites are gonna love tunneling through the underground polyiso to reach your wood framing. Yum yum tasty house. Ohio is in termite country. Instead of polyiso, you could use rigid mineral wool; insects don't eat rocks, even fluffy rocks. And if it were me, I'd go thicker than 2". 2" of continuous insulation is pretty weak for new construction in zone 5 with 2x4 walls. This is your best opportunity to add more! 4" of mineral wool + R-15 mineral wool batts between the 2x4s is pretty good insulation.

  2. user-995912 | | #2

    Hi Nick,

    A couple comments:

    7. House wrap, also called the Weather Resistant Barrier(WRB) is required to NOT be a vapor barrier.
    About the only place to use a vapor barrier is under your slab. Lots of info on this site about vapor barriers, vapor retarders etc. More on WRBs:

    8.Polyiso is NOT to be used below grade as exterior foundation insulation. For rigid foam in this application it's EPS or XPS. Preferably EPS if your set on using foam. As a builder, exterior foam makes me lose sleep at night. I have seen extensive insect damage both from termites and especially carpenter ants. For this reason I prefer insulating the inside of the foundation in my neck of the woods-VA mountains. Again, lots of great info on GBA regarding this subject.

    Also, read this re: Polyiso .

    If you go the foam route, you might consider reclaimed goods. is one source.

    Q no 3. Fiber Cement siding and wood siding wont have any issue in this application and a rainscreen is always a good thing. I can't comment on vinyl as I've never installed it.

    Good luck.

  3. wjrobinson | | #3

    Your GC needs to start studying all he can at building science corporation and here. Right from the start your plan is terrible. DIY ideas are way too experimental. As a DIYer copy a known to work set of specs.

    One suggestion for now; Don't use OSB, use Zip. Almost every builder here now does. Air seal all the other joints that the Zip tape does not. And your insulation level so far is low. Shoot for 30-40 for your walls, Shoot for r-5 windows.

  4. beining35 | | #4

    Thanks for all of the comments.

    1. Bug damage was definitely a concern of mine. After doing a little more research I think I am going to take the safe route and put the rigid foam on the interior of the basement walls. I think I can get close to the same results without the risks of bugs and other damage to the foam. I do plan to place foam below the slab also.

    2. I do agree that 2" of foam and 2x4 walls was on the small side but I wanted to start there and go more if i need to. The easier answer is always bigger or more, but this stuff does cost money and there is a point where you get no return. I think I am now planning for 4" of foam. Reasoning for this is so that I end up with the exact thickness of my basement walls. I can place the sill plate on the inside edge of the foundation then stack the 1/2 sheathing, WRB, and 4"foam, = 8".

    3. Question: has anyone used stone veneer to run a continuous wall from above grade to below grade? My house will have a walkout basement so There will be walls of foundation I want to cover with stone veneer. I will also be installing stone veneer part way up the exterior walls on some parts of the house. What would I need to do about the joint from sill plate to foundation to prevent the stone joints from cracking?

    4. Note: Sorry, I mistyped. Yes, I meant vapor retarded. Standard WRB.

    5. I do plan on use Fiber Cement siding, so I'm glad I shouldnt have to worry about warping.

    6. I will provide a cross-section view of my wall plans soon so that should help the community comment on my plan.

    Thanks again.

  5. Expert Member
    Dana Dorsett | | #5

    A few comments not fully addressed by others:

    Us sheet EPDM such as Delta Footing Barrier for the capillary break between the footer & the foundation wall- I'm not aware of a spray-on solution that really works here. As an alternative, Fab-Form's FastFoot polyethyene membrane that goes under & around the footing prior to pouring works..

    Strongly consider using insulated concrete forms (ICF) rather than putting all the insulation on the interior. Use the spray-on waterproofing for the below-grade section, and a vapor permeable purpose make material such as QuikCrete Foam Coating to protect & finish the above grade EPS: A minimalist 2" +2" ICF comes in at about R16, which meets IRC 2012 code min (R15) for insulating basements in zone 5. A more common 2.5' + 2.5" ICF is R20, and not much more expensive. With a competent ICF crew, it's quicker and cheaper to build ICF insulated foundations than it is to insulate from the interior.

    Polyiso will underperform it's labeled R-rating in insulating sheathing applications in zone 5. In a 2x4 /R15 wall the innermost inch of polyiso next to the structural sheathing will perform OK average about R5.5 during the coldest weather, but the outermost inch will average R3 or less. Using 2" of EPS (or 2.5", if you're using an R20 ICF) would be cheaper, and about the same wintertime performance. If you want to maximize performance make the outer inch EPS, and the rest polyiso.

    If installing the housewrap between the foam & sheathing, use a crinkle-type housewrap such as Tyvek DrainWrap (tm) not the flat stuff. The crinkly stuff give it a bit of capillary break, and allows the window flashing to actually drain. Placing the housewrap between the foam & sheathing also means you have to install Z-flashing at the bottom of the sheathing to direct the bulkl moisture outward, and leave at least 1/16" of space between the flashing and bottom edge of the foam so that water can get out. This creates a stripe of thermal bridging. Use copper for that Z- flashing, since the leached metal from rain that will seep into the stucco-cladding of the above grade part of ICF foam (and in the soil adjacent to the foundation) is toxic to termites & ants.

    Using vertical rather than lateral 1x4 furring to hold the foam allows you to install bug screen at the bottom & top of the 3/4" deep cavities. That way any moisture that gets by the siding can drain unimpeded, and convection will then dry what's left. This keeps the furring drier, less susceptible to fungus & mold.

  6. ohioandy | | #6

    Nick, someone else building green in Northwest Ohio? It's lonely out here. I'm in Bluffton, just finishing a small efficient house and trying for LEED certification. Super hard to find any contractors with ANY knowledge, even of modern building codes. Where I am, no inspections of any sort. Ping me at [email protected] if you want to compare notes on anything.

  7. PAUL KUENN | | #7

    G'day Nick!

    Watch during the winter and spring for super deals by the truckload on Craigs list for EPS and XPS foam. Every house we've done never cost more than $800 for a truckload of board foam and it is usually in great condition. As long as you can see it wasn't left outside in full sun for a long time it will out perform Poly Iso in the cold and save the environment from more garbage. Good luck!

  8. beining35 | | #8

    Andy hit the nail on the head.....Up in northwest Ohio there are not a lot of contractors that branch out and try new things. Most just keep to what they know. This is the main reason that I am doing all of this research. I am basically going to have to teach my contractor how to build these advanced systems. I certainly don't know everything and I'm sure he will have very knowledgeable input on the design, but he won't have time to do the research that I am doing. ICF's are a great idea, but it is not something I want to tackle on my own and to my knowledge, there is 0 experience with ICF's in my part of the country. So I'm going to steer clear.

    By the way Andy, I am building just 30-40 mins from Bluffton. I will definitely email you soon to compare notes. Interested to know what subcontractors you used for certain things. (or maybe you had the time to do it all yourself, I wish I did).

    After doing some more research and planning, here is my updated design:

    1. Poured concrete footers with capillary break.
    2. 8" concrete walls will be sprayed with flexible moisture barrier.
    3. I will insulate the basement from the inside using rigid foam.
    4. Walls will be 2x6 construction, 24” OC
    5. I plan to cover the exterior with ½” OSB sheathing
    6. Wrap this sheathing WRB that will drain
    7. Use 2” of XPS foam on exterior. Tape all seams. R-10. For the interior, I plan to install unfaced batt insulation in the 2x6 wall cavities. R-21. This is well above code and I ran rough cost numbers, and I think this is my biggest bang for the buck. I will also still get the air sealing that I want.
    8. Install 1x3 lattes 12” OC over top of the foam to act as nailers for the siding. Z channel and bug screen will be installed at the bottom. Walls are 24” OC but my lattes will be 12” OC. Every other latte will be screwed just to the sheathing, not a stud. Fiber cement siding requires less than 24” OC,
    9. I plan to have all of my roofs built as cathedral ceilings (I will only have some rooms that will actually be cathedral in the house, other rooms will have dropped ceilings with unvented attic space above.
    10. I will be installing standing seam metal roof. Under the roof, I will install WIP, then 2” polyiso on top of the deck (reflective side out), then 1x3 lattes running up the roof at an angle so that the roof can vent from top to bottom. Some type of vented screen will be installed at the top and bottom.
    11. I will then, either install batt insulation in the rafters, or used open cell spray foam.
    I’m still working on the cross section of my design, but I hope to post that soon.

  9. Expert Member
    Dana Dorsett | | #9

    Nick: Polyiso on the outer layers is going to drastically underperform in an Ohio winter, and 2" is woefully too little to use with a code-compliant total R of R49. At a code-compliant R49 in zone 5 (most of OH) it takes a minimum of R20 on the exterior, with R39 on the underside, per chapters 8 & 11 of the IRC:

    Even in zone 4 it would take R15 exterior/R34 interior to get there.

    The key is the ratio of exterior-R to total-R. If you go higher total R keep the ratio the same (and conversely.) So in most of OH you'd need about 40% or more of the total R to be exterior foam to work.

    To hit a reliable R20 that doesn't derate severely in winter you can't just use 3.5" of polyiso. But 2" of EPS on the outer layer, with 2" of polyiso between the roof deck & EPS would get you there. EPS increases performance at lower temp, polyiso falls off a cliff, but 2" of EPS is enough to keep the 2" of polyiso warm enough to stay in it's performing range. It's fine to use foil-faced EPS for that exterior layer, and beneficial from a thermal point of view if there's an air gap between the facer and the metal roofing. (Without the air gap it's no better than unfaced EPS.)

    XPS is all currently blown with HFC134a, and is something of an environmental disaster compared to EPS and polyiso, which are blown with pentane, a much more innocuous agent. As the HFCs leak out over the next 5 decades (the anticipated lifecyle of a metal roof) it's R-value falls to about the same as EPS of the same thickness, whereas EPS performance is stable over that time frame. Unless it's reclaimed goods, avoid XPS, and de-rate it to R4.2 per inch for design purposes, since that is where it will be in 50 years.

    It's fine to use the dual-foam stackup for your insulating sheathing too, and unlike 2" of XPS, 1" polyiso +1" EPS will actually still perform at R10 in 50 years, whereas the XPS would only run about R8.5 by then. The stackup would be the same as on the roof, with the EPS facing the siding, and the polyiso hugging the structural sheathing.

  10. beining35 | | #10

    I completely keep forgetting that the polyiso derates when exposed to the cold. Most all of the videos I watch and articles I read use polyiso, but now that I think about it, they are all in warmer climates like Texas.

    I now understand and like the strategy of the layered rigid foam. 2" of polyiso followed by 2" of EPS for the roof and I think I'll change my wall design to 1" of polyiso followed by 2" of EPS.

    Questions about EPS though:
    1. Is this stuff really that nasty to work with (messy and a little more fragile than the other rigid foams)
    2. Can you even buy EPS that is foil faced?
    3. Would it be ok to use unfaced EPS under the roof and behind the siding or would I have to wrap it with something? I think unfaced will be difficult to tape.

    Also, I think I'll stick with XPS under the basement slab because I think I can get reclaimed goods for this.
    I will use polyiso for basement interior walls.

    With the inexperience of contractors in my area for this type of building strategy, I think they are going to really squirm when I mention I want unvented attics. With the design i described above, does anyone have any concerns with the unvented attic? I mean it's really the same strategy as the walls, just a little more insulation. Does it make any sense to add blown-in insulation to the rooms with the dropped ceilings on the second floor? My thought is that ii might take more energy to bring the house up to temperature, because it is heating/cooling the attic space as well now, but when its at steady-state, it will hold temperature really well.


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  12. Expert Member
    Dana Dorsett | | #12

    EPS is available in a variety of facers. Taping unfaced stuff can work, but it's advisable to seal over the tape with fiber-reinforced duct mastic to retain better air-tightness for the long haul. (This is true of XPS as well, since it has a history of shrinking with age.)

    At 1.5lbs density (Type-II) EPS isn't NEARLY as crumbly as 1.0 lbs (Type-I) or lower. A lot of Type-I is sold with facers to keep it from getting dinged up in handling.

  13. Irishjake | | #13

    For answers to almost all your questions check out these articles, and some of my comments at the bottom. Lots of reading, but before you make any decisions I suggest reading them all, and when you've read most or all of them you'll probably find that your house will look a lot like this one:

    You'll notice that Martin Holladay is the author of quite a few of these articles too!

    These will give some further understanding too:

    A few ideas to consider -
    Instead of just plain EPS, try going with NEOPOR (graphite impregnated EPS), but because it is so new you'll never find any on the recycled market. Neopor along with regular EPS can be purchased from Perma-R Products, with or without a facer or reflective facer in almost any thickness and length you want.

    If you're going to go with recycled insulation - Insulation is a great resource, and there are surely some recyclers near you too that resell recycled rigid foam. Insulation Depot can source it from nearby you too though.

    A note on R-value: Insulation R-values are deceiving - the R-value of insulation is always advertised at what it's tested at (room temperature of 70 deg F). and a lower temperature - Who needs insulation @ 70 deg F?!!!, so always try to find the published R-value of insulation at its tested lower temp -especially if you live in the zones 5-6+)

    Poly-iso suffers from a drop in R-value as it gets colder - XPS and EPS/NEOPOR do not - their R-value goes up. Poly-iso and XPS both shrink more than EPS. Poly-Iso and XPS both also use blowing agents that increase greenhouse gas emissions. Poly-iso should never be used below grade. XPS holds water when used below grade, and although is more resistant to water than EPS, does not shed the water like EPS does. This water retention of XPS causes it to lose R-value overtime. If you soak XPS in a bucket and EPS - EPS will dry - XPS won't dry for a long, long time. EPS is less expensive.

    Think EPS all around. Exterior walls, below grade, sub-slab. You can get high psi EPS for sub-slab. Put the EPS in the middle of your foundation walls to gain the biggest benefits. You create a mass wall on the inside (which increases the thermal performance of the whole foundation wall assembly), protects the insulation from critters and insects, and allows you to use the inside face of your foundation as a finished wall. Use fiberglass form ties to reduce thermal bridging through your foundation wall, and prevent the need to fill every tie hole that is left after the forms are removed, and reduce iron bleeding from the form tie on the interior wall. (Check out SuperTie or Fibre-Tie)

    You can use a vapor permeable or a non permeable membrane on the exterior face of your sheathing too - just don't use one on the inside.

  14. Irishjake | | #14

    Make sure to check on the spacing pattern of the rainscreen 1x3 for your fiber cement too. There is no reason you can't install it 24" OC. The 1x3 should not be installed into just sheathing, and would be a waste of your resources to do such a thing. Check out the architectural drawings on this link which refers to the same house I referred in my last post. It will give your further info behind the design considerations that were made too, which can be really insightful.

    good luck!

  15. Dana1 | | #15

    Brad writes:

    "A note on R-value: Insulation R-values are deceiving - the R-value of insulation is always advertised at what it's tested at (room temperature of 70 deg F). and a lower temperature - Who needs insulation @ 70 deg F?!!!, so always try to find the published R-value of insulation at its tested lower temp -especially if you live in the zones 5-6+) "

    This is a common misconception, but the ASTM C518 protocol for R-value labeling does NOT test at room temperature. It tests at a temperature difference across the foam of 30F, with one side at 90F, the other at 60F, the mean temp through the foam being 75F. Neither the warm side nor the cold side are at room temperature.

    When an EPS manufacturer specifies a 40F or 25F performance, they've moved the bracket down, but kept the 30F delta-T. A 40F spec for Type-II EPS is typically R4.5 @ 1". but that's with the cold side at 25F, and the warm side at 55F. Similarly, a 25F spec could be R4.7 @ 1", which means the cold side is +10F, the warm side 40F.

    What those numbers clearly are NOT is the performance per inch at some outdoor temperature, with the interior of the house being at room temperature.

    Polyiso derating is very non-linear and varies by manufacturer, but generally speaking layers of polyiso that average above 35F (mean temp through the layer) will outperform EPS inch-for-inch, but below that EPS will outperform the polyiso. The highest performance stackups depend also on the R-value of any fiber insulation (on either side of of the foam),. For 2" outsulation in zones 5 & 6 on a fiber-filled 2x4 wall, an all EPS solution is NO the highest performance for a given total wall thickness- a dual-layer combination with EPS on the outer/coldest layers with polyiso inside of that yields higher performance than either material alone. For a simplified non-dynamic analysis it's useful to look at the average temperatures through the layers at the binned hourly wintertime mean temperature for a given location.

    At typical zone 5 location might have a January outdoor mean temperature of +25F. The crude arithmetic on the layers of EPS/polyiso/cavity fiber at a static indoor temp of 70F, means guesstimating the EPS and polyiso up/down rating based on the approximate average temp through the layer. (Even this crude model requires a fair amount of arithmetic.) In the zone 5 case with a 25F outdoors, 70F indoors (a 45F total delta), assuming R13 cavity fill an outer 2" of EPS would perform at about R4.6/inch (since it's mean temp through the layer s somewhere between 25-40F) for R9.2. With R13 cavity fill that's a total of R21.2 at center cavity, at a 45F delta-T, or about 2.1 F/R. With R13 cavity fill it means the temp at the sheathing will then average about 43F, and the mean temp through the foam is about 35F. If you swap the inner inch of EPS for polyiso, the polyiso being above 35 even on the cold side means it will outperform the EPS slightly, even in January. But during months warmer than January it will outperform an inch of EPS in that intermediate layer by quite a bit, which relevant on both a dew-point control and energy use basis.

    Note that at the framing the temperature of the interior side of the foam is higher still (due to the very low ~R4.2 of the framing fraction, 25% of the overall wall area), which means that the intermediate layer of polyiso will be performing in a substantially higher-R range for the thermal break over the framing than the all EPS solution would.

    Note- you don't need high-psi Type-IX or Type-X IV EPS for sub-slab foam- Type-II is fine (typically 15psi). Even Type-I is fine from a compression point of view but it can take on more moisture than Type-II. The only time you need to engineer the foam for the compressive load is when installing it under footings, where it is supporting the load of the entire house. You could park your D9 Caterpillar on a 4" slab over Type-II EPS without risk of breaking the slab.

  16. Irishjake | | #16

    Dana....thanks for the corrections and obviously more knowledgeable answer - just when I think I have a thorough understanding of building science - you throw Delta T's across the room! Ugh......

  17. Expert Member
    Dana Dorsett | | #17

    If there isn't a temperature difference (delta-T) across the foam, it's R value is irrelevant at ANY temperature, eh? :-)

    But it is pretty confusing when they throw out just the mean temperature of the foam along with an associated R-value, unless you understand the full test conditions and what that temperature really is (the mean temp through the foam layer, not the outside temp.) Some ICF vendor marketing cites the 40F R-values for EPS without really explaining that it would only be valid with a 25F outside temp and a 55F interior temp. It'll have approximately that performance at +10F outdoors & 70F indoors (a 60F delta-T, not the as-tested 30F delta) but +10F is not a common wintertime mean outdoor temp in most of the US (but in climate zone 7 it is.) While it's not an outright lie, it citing that number without the asterisk with the more complete explanation seems designed to deceive, since many people would assume that to mean a 40F outdoor temperature. A 40F outdoor temp with a 70F interior would be a 55F mean temp through the foam with a 30F delta, but that isn't a commonly tested temp.

  18. Andrew_C | | #18

    Vented attics -
    Vented attics are simple, cheap, and robust compared with most unvented designs. Every builder knows how to build them (although you'll have to make sure raised heel trusses are incorporated). You can pile cheap, environmentally benign (relatively) cellulose as high as you want. I don't understand all the people who continue to use unvented designs if they have a clean sheet to start the project. Even people who know what they are doing wrt insulation, moisture, etc still screw up (see Lstiburek's articles on his own) unvented roofs. IMO, using foam in the roof deck assembly is experimental and asking for trouble, especially if you are convinced that local builders do not have any experience with the sort of detailed work that this seems to require to have a chance at success. And, if you have a full basement, you don't need any additional room that might be gained by having an unvented attic.

    There seem to be all sorts of articles or Q&As on GBA and BSC about how to build or fix cathedral ceilings and/or unvented attics. There are a lot of different problems, and a lot of different proposed (experimental, IMO) solutions. All of which could be avoided by sticking with a proven vented roof design that works everywhere.

    Nick (OP): this is kind of a personal rant against most approaches to unvented attics and is not directed at you. Building your personal house should be a very interesting and rewarding experience, and I wish you the best of luck. I do understand how much effort it takes to tease out the good ideas that you want to use from vast trove of information here at GBA (let alone FineHomeBuilding and BSC).

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